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Optimal f-stop for 1:1 reproduction scale

Last Updated: 27th Dec 2010

Setting the optimal aperture in close up range at 1:1 reproduction scale for mineral photography



By Volker Betz, Taunusstein

Macro lenses for DLSR cameras allow, without any additional extension tube, bellows or add on lenses, a reproduction scale of 1:1.

As sharpness is essential and depth of field is narrow, some experiments have been made to find the optimal aperture (f-stop) for a Canon EF-S 60 mm f/2.8 Macro USM lens attached on a EOS 450 D camera body. At 1:1 reproduction scale the field of view is about 15x22 mm.

It is a old and common thumb of rule, that photographic lenses have a maximal sharpness at an f-stop of at about double the open aperture. For the lens tested here so maximum sharpness would be expected at f= 5.6.

For that purpose a millimetre scale made from Aluminium was photographed in a 45 ° angel as close as possible with the f-stops of 2.8, 4.0, 5.6, 8.0, 11,16, and 22. The focus was set at the 5 cm mark and from the picture the depth of field estimated. As the scale is seen from a 45 ° angel the scale has to be divided by √ 2 ( 1.41) so the visible field is 22 mm. The depth of field has to calculated the same way. Also a 2 mm fraction from the picture at the 5 mm mark was cropped and the sharpness compared by visual inspection. A scratch in the scale helps to distinguish small differences.


Sharpness and depth of field



As it can seen from the picture above, best sharpness is obtained at an f-stop of 8, slightly more closed than expected. More closing gradually causes a loss of resolution and gains depth of field. As a conclusion for myself, f-stops of more closed than 11 should not be used at that reproduction scale. Better results are possible by an f-stop of 8. As depth of field is only about 3 mm under that conditions, layer composition is the better way to enhance depth of field.




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Comments

Hi Volker,

Excellent article. The f11 - f22 images illustrate the empty magnification that comes from diffraction effects beautifully. When I used to do film photography, I would use f5.6 and f8 depending on the total tube length. The key was to not exceed f45 in the total system. These days with digital imaging, image stacking and other techniques, depth of field is nearly neglected.

Henry

Henry Barwood
28th Dec 2010 1:33am
Hello Volker & Henry.....in photo school MANY years ago, I was taught the rule that the best compromise between sharpness, DOF and distortion occurs "one stop down (closed) from the middle". In your example, f11 seems sharper over-all - edge and center sharpness seem equal & you've gained a couple of mms in the depth of field. I zoomed the image to 200% and would happily use f8 or f11 with your lens - it's a fine Macro!!!

Don Saathoff
29th Dec 2010 12:24am

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